Tendency to form pentahalides decreases down the group 15 of the periodic table. Account for this observation.

Explanation

Due to absence of d-orbitals, N does not form a pentahalide but forms only a trihalide due to the presence of three half-fille 2 p-orbitals. All the remaining elements (i.e., P, As, Sb, Bi, however form both tri-and pentahalides due to presence of three p-orbitals as well as vacant d-orbital in the valence shell). But due to inert pair effect, tendency of As, Sb and Bi to form pentahalides decreases while that of trihalides increases. In other words, the tendency to form pentahalides decreases down the group, i.e., `PX_(5) gt AsX_(5) gt SbX_(5) gt BiX_(5)`. Further, the stability of pentahalides of any particular element decreases as the size of the halogen (X) increases from F to I. Thus, the only stable pentahalide of Bi is `BiF_(5)`.